The deaths of four people in the space of a day are believed to be linked to heroin, police have said.
Three men and a woman were found dead at separate addresses in the Barnsley area on Good Friday.
South Yorkshire Police said it was not officially linking them at present but warned drug users to exercise caution.
Temporary Ch Insp Ian Proffitt said the force was trying to establish a possible link to the strength or content of heroin being sold locally.
"For four deaths to occur in similar circumstances in a small time period and in a relatively small geographical area is unusual," he said.
"The public should exercise caution if they come into contact with controlled drugs, particularly heroin, or heroin derivatives.
"If you experience any unusual symptoms after taking drugs, seek medical attention immediately."
Two men, aged 33 and 40, were found dead at separate addresses in Barnsley on Friday morning, while a 47-year-old woman was found dead at a house in the village of Grimethorpe.
A third man, a 31-year-old, was found dead at an address in Barnsley on Friday evening.
The force said it was awaiting the results of toxicology tests.
A 37-year old man and a 42-year-old man arrested on suspicion of supplying controlled drugs have been released on bail, it added.
South Yorkshire Police is urging anyone with information about the illegal supply of drugs to contact them.